The corrosion of metals begins with the adsorption of small amounts of water onto metal surfaces. Wetting provides the means for transport of environmental acids, halides, and other corrosive materials. A hydrophobic surface which is water repellent inhibits the adsorption of environmental moisture and significantly reduces corrosion of a plated metal deposit and the underlying layers or substrate.
Fluorinated polymers such as polytetrafluoroethylene (marketed under the trade name TEFLON®) are known for imparting hydrophobicity on a surface and thus imparting water repellency. Fluorinated polymers are typically applied to a metal surface as particles, which are sintered together by baking at high temperatures.
Recently, methods have been developed to directly deposit fluorinated polymer particles into a metal-based composite coating, which avoid the high temperature sintering of fluoropolymer particles. For example, Henry et al. (U.S. Pat. No. 4,830,889) and Feldstein (U.S. Pat. No. 5,721,055) describe the co-deposition of fluorinated polyethylene and nickel from electroless nickel plating baths. See also Kobayashi et al. (U.S. Pat. No. 6,878,461).
As distinguished from electroless processes, Abys et al. (U.S. Pat. No. 6,274,254) disclose a method for co-depositing Pd, Co, and PTFE by electrolytic plating to increase the wear resistance of electrical connectors.
An electrolytically plated metal-based composite coating comprising PTFE particles having diameters comparable to the wavelengths of visible light (i.e., from about 380 nm to about 780 nm) produces a dark gray, matte surface. Such alloys make poor finishes in decorative and electronics applications, such as, for example, decorative automotive parts and electronics connectors, where surface appearance is either desired for cosmetic reasons or required for performance such as wear resistance. Additionally, composite coatings comprising relatively large PTFE particles repel water to varying degrees based on the particle size distribution, concentration of occluded particles, and surface to volume ratio of the particles that are incorporated. Moreover, large size particles may not be evenly distributed in the metal-based composite coating.
Therefore, a need continues to exist for a plating process which yields a metal-based composite coating having a smooth, bright, glossy finish, a high degree of water repellency and corrosion resistance, and a lubricious surface that improves wear resistance without affecting the appearance of deposit.